SHAPE PRESERVING CHEMICAL TRANSFORMATION OF ZnO MESOSTRUCTURES INTO ANATASE TiO2 MESOSTRUCTURES FOR OPTOELECTRONIC APPLICATION

ABSTRACT

The present application discloses a shape preserving chemical transformation of ZnO mesostructures into anatase TiO 2  mesostructures using controlled low temperature TiCl 4  treatment for optoelectronic applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to and claims the benefit of IndianPatent Application No. 2220/DEL/2011, filed Aug. 5, 2011, whosedisclosure is hereby incorporated by reference in its entirety into thepresent disclosure.

FIELD OF THE INVENTION

The present invention relates to shape preserving chemicaltransformation of ZnO mesostructures into anatase TiO₂ mesostructuresusing controlled low temperature TiCl₄ treatment for optoelectronicapplications.

BACKGROUND AND PRIOR ART OF THE INVENTION

Titanium dioxide (TiO₂) is perhaps one of the most widely usedtransition metal oxides in diverse applications due to its variety ofunique and application-worthy properties. With the growing emphasis ofthe current science and technology on nanomaterials due to their uniqueand novel property domains, considerable efforts have been expended overthe past decade to synthesize various metal oxides (including TiO₂) inthe form of different phases, shapes, and functions using a variety ofsoft chemical and physical synthesis techniques. Depending on the use ofa particular process, specific precursors/radicals, capping agents,temperature, pressure etc. a particular morphology of the nano systemevolves. In the context of different applications such as photovoltaic,catalysis, electro-optics etc., controlled nanocrystal growth isintensely researched. In addition to the size and composition, the shapecontrol of nanomaterials is an important variable to adapt to theproperties for various applications. However, different oxides havetheir specific symmetry-dependent crystal growth habits which make theproposition of developing specific desired (shape) morphology anon-trivial proposition. For instance, ZnO can grow more easily intoanisotropic structures while TiO₂ does not, unless efforts are made forfacet control via selective capping.

Literature survey shows that the chemical transformation of inorganicnanocrystalline solids via diffusion or exchange of atoms is emerging asan attractive approach for nanostructure engineering in recent years. Inparticular, for transforming one ionic nanocrystal into anotherhetero-interfaced nanostructure, cation exchange reaction is shown to bea very useful process. It is generally assumed that the anionicstructure of the crystal is conserved, while the cations undergoreplacement during the exchange reaction due to their relatively smallersize and higher mobility. For instance, the morphology composed of aCdSe nanocrystal embedded in a CdS rod (CdSe/CdS) was exchanged to aPbSe/PbS nanorod via a Cu₂Se/Cu₂S structure keeping the seed size andposition within the nanorod preserved. The morphology change in thecation exchange reactions of metal chalcogenide nanocrystals, CdE toMxEy (E=S, Se, Te and M=Pd, Pt) has been investigated by Son et al.Brock et al. have synthesized Ag₂Se wet gel monoliths by an ion exchangereaction of a monolithic CdSe wet gel and converted the same to anaerogel by drying under supercritical conditions.

TiCl₄ treatment of nanoparticulate TiO₂ films has been researched byseveral groups, especially by O'Regan and Bakker, in the context of thedye sensitized solar cell (DSSC) application, and a significantimprovement in cell efficiency has been demonstrated following such atreatment. However, there have not been many studies on the possiblebeneficial use of such a treatment for other oxides.

ZnO has attracted considerable interest of the DSSC community due to itsunique set of optoelectronic properties; however, the corresponding DSSCefficiencies are quite low. The pioneering work by Yang and coworkersshowed that DSSCs based on ZnO nanowire/TiO₂ core-shell structures havehigher charge separation yields. It is now known that TiO₂ coating ofZnO nanostructure improves the DSSC efficiency, and in most cases suchcoating is applied by the expensive atomic layer deposition method. Onlyrecently, Atienzar et al. reported a simple TiCl₄ treatment that led thesurface coating of TiO₂ on ZnO core (equivalent of the TiCl₄ posttreatment of TiO₂ structured materials) leading to improved DSSCperformance. However, no details were provided about the effects ofTiCl₄ on ZnO morphology. Recently, the effect of TiCl₄ treatment onporous ZnO photoelectrode has also been examined by Murakami et al.

An article titled “Materials “Alchemy”: Shape Preserving ChemicalTransformation of Micro-to-Macroscopic 3-D Structures” by Kenneth H.Sandhage, published in TMS Vol. 62, No. 6 (2010) pp. 32-43 gives anoverview on Shape Preserving Chemical Transformations. The articlestates;

-   -   “The scalable fabrication of nanostructured materials with        complex morphologies and tailorable chemistries remains a        significant challenge. One strategy for such synthesis consists        of the generation of a solid structure with a desired morphology        (a “preform”), followed by reactive conversion of the preform        into a new chemistry. Several gas/solid and liquid/solid        reaction processes that are capable of such chemical conversion        into new micro-to-nanostructured materials, while preserving the        macroscopic-to-microscopic preform morphologies, are described        in this overview.”

An article titled “Synergistic effects of ZnO compact layer and TiCl₄post-treatment for dye-sensitized solar cells” by NiuHuanga et al.,published in Journal of Power Sources, Volume 204, 15 Apr. 2012, Pages257-264 discloses the interaction between ZnO compact layer and TiCl₄post-treatment on TiO₂ photo electrode for dye sensitized solar cell(DSSC). Photo electrode combined the two modifications is designated asZnO+21+TiCl₄. It is further disclosed that after the TiCl₄ treatment theZnO compact layer transforms to a bi-functional layer, which suppressesback electrons transfer from FTO to electrolyte and reduces the FTO/TiO₂interfacial resistance. In addition, the newly formed TiO₂ coatinggenerated by TiCl₄ post-treatment contains abundant and well dispersedZn element, which further facilitates electron transfer at TiO₂ layer.Meanwhile, the electron lifetime in ZnO+21+TiCl₄ is the longest.Consequently, the overall energy conversion efficiency of the cell withZnO+21+TiCl₄ is significantly enhanced to 8.9%, which is 8.8% higherthan that with pure TiCl₄ post-treatment and 17.7% higher than thatwithout any treatment.

An article titled “Fabrication of TiO₂ nanotube film by well-aligned ZnOnanorod array film and sol-gel process” by J Qiu et al. published inThin Solid Films (2007), Volume: 515, Issue: 5, Pages: 2897-2902discloses high density TiO₂ nanotube film with hexagonal shape andnarrow size distribution which was fabricated by templating ZnO nanorodarray film and sol-gel process. Well-aligned ZnO nanorod array filmsobtained by aqueous solution method were used as template to synthesizeZnO/TiO₂ core-shell structure through sol-gel process. Subsequently,TiO₂ nanotube array films survived by removing the ZnO nanorod coresusing wet-chemical etching. Polycrystalline anatase TiO₂ nanotube filmswere similar to 1.5 μm long and similar to 100 nm in inter diameter witha wall thickness of similar to 10 nm.

Inspite of the above disclosures in the art for chemical conversion intomicro-to-nanostructured materials, while preserving themacroscopic-to-microscopic preform morphologies, chemical transformationof ZnO mesostructures to TiO₂ mesostructures using simple chemicaltreatment is however not explored hitherto.

In the context of the importance of applications of shape controlledmetal oxides in electro-optics, photovoltaics etc. the present inventionlays emphasis in providing shape preserving chemical transformation ofZnO mesostructures to TiO₂ mesostructures using simple chemicaltreatment.

OBJECTS OF THE INVENTION

Main object of the present invention is to provide chemicaltransformation of ZnO mesostructures to anatase TiO₂ which exhibits aremarkable nominally shape-preserving property.

Another object of the present invention is to provide shape preservingchemical transformation of ZnO mesostructures into anatase TiO₂mesostructures using controlled low temperature TiCl₄ treatment foroptoelectronic applications.

SUMMARY OF THE INVENTION

Accordingly, present invention provides a process for the shapepreserving chemical transformation of ZnO mesostructures into anataseTiO₂ mesostructures comprising the steps of:

-   -   i. treating the Zinc oxide mesostructures with Titanium        tetrachloride (TiCL₄) solution at temperature in the range of 60        to 70° C. for period in the range of 20 to 30 min;    -   ii. annealing the TiCl₄ treated Zinc oxide mesostructures as        obtained in step (i) at a temperature in the range of 400 to        450° C. for period in the range of 20 to 30 min to obtain        anatase TiO₂ mesostructures.

In an embodiment of the present invention, Zinc oxide mesostructures areselected from the group consisting of Zinc oxide rods, Zinc oxidespheres, Zinc oxide flakes and Zinc oxide flowers.

In yet another embodiment of the present invention, the Zinc oxidemesostructures are coated over Titanium dioxide nanoparticles film andannealed at a temperature in the range of 400 to 450° C. for 50 to 60min before treating with Titanium tetrachloride (TiCl₄) solution.

In yet another embodiment of the present invention, the Zinc oxidemesostructures are optionally grown on Fluorine doped Tin oxide (FTO) orIndium doped Tin oxide (ITO) glass plates before treating with Titaniumtetrachloride (TiCl₄) solution.

In yet another embodiment of the present invention, the Zinc oxidemesostructures treated with Titanium tetrachloride (TiCl₄) solution arewashed with deionized water.

In yet another embodiment of the present invention, the thickness ofanatase TiO₂ mesostructures is in the range of 5-12 μm.

In yet another embodiment of the present invention, the diameter ofanatase TiO₂ mesostructure is ranging from 500 nm to 2 μm.

In yet another embodiment of the present invention, anatase Titaniumdioxide mesostructures prepared by the aforesaid process.

In yet another embodiment of the present invention, anatase Titaniumdioxide mesostructures are useful for optoelectronic applications.

In yet another embodiment of the present invention, the dye sensitizedsolar cells utilizing said mesostructures exhibit efficiency in therange of 3.5% to 7%.

In an embodiment, present invention provides a process for the shapepreserving chemical transformation of ZnO mesostructures into anataseTiO₂ mesostructures with remarkable nominally similar shapes comprisingthe steps of;

-   -   a. providing films of ZnO mesostructures grown on fluorine doped        tin oxide (FTO)/Indium doped tin oxide (ITO) glass plates;    -   b. treating films of ZnO mesostructures as provided in step (a)        with TiCl4 solution at temperature in the range of 60 to 70° C.        for period in the range of 20 to 30 min; and    -   c. washing the treated films of step (b) with de ionized water        followed by annealing at temperature in the range of 400 to        450° C. for period in the range of 20 to 30 min to obtain        anatase TiO2 mesostructures.

Alternatively, in another embodiment, present invention provides aprocess for the shape preserving chemical transformation of ZnOmesostructures into anatase TiO₂ mesostructures with remarkablenominally similar shapes optionally comprising the steps of;

-   -   1. coating a layer of ZnO mesostructures over TiO2 nanoparticles        film;    -   2. annealing the films of step (i) at temperature in the range        of 400 to 450° C. for period in the range of 50 to 60 min;    -   3. treating the films of step (ii) with TiCl4 solution at        temperature in the range of 60 to 70° C. followed by second        annealing at temperature in the range of 400 to 450° C. for        period in the range of 20 to 30 min to obtain anatase TiO2        mesostructures.

BRIEF DESCRIPTION OF FIGURES

FIG. 1: XRD data of ZnO Flower (Flr), TiO₂ and TiCl₄ treated ZnO Flower(Fir).

FIG. 2: Raman spectra of TiCl₄ treated ZnOFlr and ZnOFlr (inset).

FIGS. 3( a) and (b): The respectively SEM images of ZnOFlr and TiCl₄treated ZnOFlr The inset FIG. 3( b) is the zoomed-in version of one ofthe flowers of TiO₂ (TiCl₄ treated ZnOFlr).

FIG. 4( a) FE-SEM of ZnO rods, (b) TiCl₄ treated ZnO rods on FTO and (c)schematic diagram of mechanism involving conversion of ZnO rods to TiO₂hollow tubes by TiCl₄ treatment of ZnO rods.

FIG. 5: FE-SEM (Field Emission Scanning Electron Microscope) of ZnOflakes (a) and TiCl₄ treated ZnO flakes (b) on FTO.

FIG. 6: Comparison of Solar cell characteristics for DSSCs made with thenanocrystalline TiO2-layer-based TiCl₄ treated ZnOFlr over layer withNanocrystalline TiO₂ and commercial Degussa P25.

FIG.-S-1: XRD data of (a) ZnO (Rods, Spheres, Flakes), (b) TiCl₄ treatedZnO (Rods, Spheres, Flakes).

FIG.-S-2: Raman Spectra of (a) ZnO (Rods, Spheres and Flakes) and (b)TiCl₄ treated ZnO (Rods, Spheres and Flakes).

FIG. S-3: Energy dispersive x-ray (EDX) data of TiCl₄ treated ZnO.

FIG. S-4: XPS (X-ray photoelectron spectroscopy) data of (a) ZnO andTiCl₄ treated ZnO, (b) Presence and absence of Zn in ZnO and TiCl₄treated ZnO respectively, and (c) Presence of Ti in TiCl₄ treated ZnO₅.

FIG. S-5: SEM Data of ZnO rods (a) & (c) and TiCl4 treated ZnO Rods (b)& (d) on ITO.

FIG. S-6: SEM Data of ZnO capped with PVP (a) & (c) and TiCl4 treatedZnO capped with PVP (b) & (d) on FTO.

FIG. S-7: Optical Absorption of solutions containing dye detached fromdoctor bladed films of different cases of interest (film area of 1.6 cm₂dye extracted in 10 mL of 1 mM KOH).

FIG. S-8: Diffused reflectance spectra of the nanocrystalline TiO₂ andTZFT films (a) without and (b) with adsorbed N-719 dye.

DETAILED DESCRIPTION OF THE INVENTION

Present invention describes a process for shape preserving chemicaltransformation of ZnO mesostructures into anatase TiO₂ mesostructureswith remarkable nominally similar shapes using controlled lowtemperature TiCl₄ treatment. The chemical transformation of ZnOmesostructures by TiCl₄ treatment results into anatase TiO₂ withoutchanging its size and shape.

Accordingly, the present invention describes a process for shapepreserving chemical transformation comprising treating ZnO mesostructurefilms grown on FTO/ITO glass plates with TiCl₄, followed by washing andannealing to obtain structured material of anatase TiO₂. The presentinvention also provides the synthesis of various ZnO mesostructures byhydrothermal and co-precipitation methods and their remarkable andcomplete transformation into anatase TiO₂ mesostructures with remarkablenominally similar shapes using controlled low temperature TiCl₄treatment.

The ZnO mesostructures grown on FTO/ITO glass plates is obtained bydoctor blade method.

The present invention describes the synthesis of various ZnOmesostructures in the form of rods, spheres, flakes and flower-likemorphologies by hydrothermal and co-precipitation methods.

The present invention describes the application of these converted TiO₂mesostructures for light harvesting in Dye Sensitized Solar Cells (DSSC)for enhanced Optoelectronic Applications.

In the geometric hierarchy, there are basically three different levelsof scales, namely, the macrostructure level, the mesostructure level,and the microstructure level. The macrostructure level represents thegross surface geometry, typically expressed as a polygonal mesh orparametric spline surface. The microstructure level involves surfacemicrofacets that are visually indistinguishable. The mesostructure levelrepresents geometric details that are relatively small but stillindividually visible such as bumps or dents on a surface. Efficientmesostructure reconstruction methods can contribute greatly tohigh-quality graphics models in terms of fine scale surface geometricdetails.

The process for the shape preserving chemical transformation of ZnOmesostructures into anatase TiO₂ mesostructures with remarkablenominally similar shapes comprises;

-   -   1. providing films of ZnO mesostructures grown on FTO/ITO glass        plates;    -   2. treating films of ZnO mesostructures of step (1) with TiCl4        solution at 70° C. for about 30 min; and    -   3. washing the treated films of step (2) with D.I. water        followed by annealing at 450° C. for 30 min to obtain anatase        TiO2 mesostructures.

The films employed in the process may be obtained by growing ZnOmesostructures directly on FTO and/or ITO glass plates as substrates ormay be prepared from nanoscale powder samples on FTO by doctor bladingmethod.

The thickness of the film obtained by growing ZnO mesostructuresdirectly on FTO and/or ITO glass plates as substrates is 5 μ.

The basic exchange reaction operative in the conversion of ZnO to TiO₂is shown in Scheme 1.

Scheme 1: Reaction involved in chemical transformation of ZnO to TiO₂

2ZnO+TiCl₄.2ZnCl₂+TiO₂

The reaction occurs via cation exchange between Zn²⁺ and Ti⁴⁺ ions.These exchange reactions can be qualitatively understood in terms ofhard-soft acid-base theory (soft acids react faster and form strongerbonds with soft bases, whereas hard acids react faster and form strongerbonds with hard bases, all other factors being equal). Ti⁴⁺ is a harderacid than Zn²⁺. Thus Ti⁴⁺ binds strongly with the O²⁻ anion to formTiO₂. The conversion of ZnO structured material to TiO₂ structuredmaterials is strongly favored by a thermodynamic driving force of about−249 kJ/mole.

Alternately, doctor blading method is used to make TiO₂ nanoparticlefilm over which a layer of ZnO mesostructures (flowers) are coated toobtain film of thickness of about 15 μm which is subjected to annealedat 450° C. for 60 min and these films are further treated with TiCl₄solution at 70° C. followed by second annealing at 450° C. for 30 min toobtain TiO₂ mesostructures (flowers) of the thickness of 11 μm.

The thickness of the film obtained by doctor blading method is ˜12 μmwhich is reduced to ˜7 μm after TiCl₄ treatment. The reduction ofthickness after TiCl₄ treatment can be attributed to some solublereaction products, which get washed out during the process.

The ZnO mesostructures of the present invention are in the form of rods,spheres, flakes and flower-like morphologies. The chemicaltransformation of ZnO mesostructures by TiCl₄ treatment results intoanatase TiO₂ without changing its size and shape. The present inventionprovides process for the preparation of ZnO mesostructures byhydrothermal and co-precipitation method.

1. Preparation of ZnO Flowers

ZnO flowers are synthesized by hydrothermal route using high purity zincacetate and NaOH. For obtaining ZnO flower aqueous solution of zincacetate is prepared and magnetically stirred. After the dissolution ofzinc acetate, aqueous solution of NaOH is added to the above solution.This solution is transferred into a Teflon lined stainless steelautoclave. It is then sealed and maintained at 180° C. for 2 h. Afterthe reaction a white colored solid powder is recovered by centrifugationfollowed by washing with distilled water and ethanol to remove theresidual ions in the final product. Then the powder (ZnO Flowers) isfinally dried in air.

Preparation of ZnO Rods on FTO and ITO

Zinc acetate, Zinc nitrate, Hexamethylenetetramine (HMT) and SodiumHydroxide Pellets are used as precursors for ZnO rod growth. Zincacetate solution is prepared in methanol and is kept under stirring.Then sodium hydroxide solution (prepared in Methanol) is added drop wisetill the solution attained slight milky color and is used as seedsolution. Fluorine doped tin oxide (FTO) and Indium doped tin oxide(ITO) glassplates are used as substrates for growth of ZnO rods. Thesubstrates are mounted on the spin coater having a preset rotation speedof 2500 rpm for 30 sec and then spin coating is carried out usingfreshly prepared seed solution. The process is repeated continuouslyuntil the transparent substrate turned slightly opaque. Finally thesubstrates are annealed for better adherence of ZnO nanoparticles whichact as nucleating sites for the growth of ZnO rods.

For facile growth of ZnO rods, equimolar solutions of Zinc Nitrate andhexamethylenetetramine are separately prepared using de-ionized water assolvent. The seeded substrates are immersed into the solution and thesolution temperature is maintained at 95° C. under slow stirring. Thedepositions are carried out for time duration of 3 hour. Finally, thedeposits are annealed for removal of moisture and for improving theadhesion.

2. Preparation of ZnO Spheres and Flakes

The ZnO sphere like morphology is synthesized at room temperature byco-precipitation method using zinc acetate, polyvinyl pyrrolidone andsodium hydroxide. Aqueous solution of zinc acetate is prepared andmagnetically stirred. Polyvinyl pyrrolidone (capping agents) isdissolved in de-ionized water and added to the zinc acetate solution.Then aqueous solution of NaOH is added drop wise to the above solution.A white colored solid powder is obtained and recovered by centrifugationfollowed by washing with de-ionized water. Then, the powder is finallydried in air. The ZnO flakes were obtained from a commercial source(Smart NanoZ, Pune, India).

3. Preparation of TiO₂ Nanoparticles

Nanocrystalline TiO₂ is prepared by using a simple hydrothermal method.Titanium Isopropoxide is hydrolyzed by adding de-ionized water and thensonicated for 5 minutes. The solution is transferred to Teflon linedautoclave vessel along with H₂SO₄. This autoclave vessel is kept for 24Hrs. The resulting product is washed thoroughly with de-ionized waterand dried in a dust proof environment to produce the powder of TiO₂nanoparticles.

Results and Discussions

FIG. 1 shows the X-ray diffraction (XRD) patterns of ZnO Flower (Fir)and TiCl_(a) treated ZnOFlr film on the glass substrate. As can be seenfrom the XRD pattern, the 2θ values at 31.8, 34.4, 36.3, 47.6, 56.6,62.8, 67.9 and 69.2 correspond to wurtzite ZnO (PCPDFWIN #800075). TheXRD data for the case of TiCl₄ treated ZnO show clear signatures of pureanatase TiO₂ phase (PCPDFWIN #211 272) at 25.3, 37.9, 48.2, 54.1, 55.2and 62.9. The complete absence of ZnO peaks clearly indicates thatfollowing the stated TiCl₄ treatment, the ZnO phase converts fully toanatase TiO₂. Several other morphologies such asspheres, flakes etc.were also studied and the corresponding XRD data are presented in FIG.S-1. These too exhibit complete transformation to anatase TiO₂ form.

The Raman spectrum for ZnOFlr and TiCl₄ treated ZnOFlr is shown in FIG.2. The Raman spectrum for the case of ZnO in FIG. 2 (inset) shows theclear signatures at about 376 and 435 cm⁻¹ expected for this oxide. TheRaman peaks at 148.2, 401, 518 and 642 cm⁻¹ in FIG. 2 are characteristicof pure anatase TiO₂ phase. No peak corresponding to ZnO was observedafter TiCl₄ treatment of ZnO, which indicates complete conversion fromZnO to anatase TiO₂ by the stated TiCl₄ treatment. The Raman spectra ofother TiCl₄ treated ZnO structured materials are shown in FIG. S-2.

FIGS. 3( a) and (b) represents the SEM images of ZnOFlr film and TiCl₄treated ZnOFlr film, respectively, which clearly confirms that themorphology remains nominally intact after the TiCL₄ treatment of ZnOFlr.Interestingly the latter TiCl₄ treated case for which XRD shows pureanatase TiO₂ phase is seen to retain the flower like morphology of theparent ZnO mesostructure, implying a nominally shape preserving chemicaltransformation. It is observed that in FIG. 3( b) the necking betweenthe flowers takes place after the TiCL₄ treatment of ZnOFlr which ishelpful for the transport of electrons in DSSC as discussed later. Suchdense branched hierarchical morphology is of great value in nanoparticlefilms for solar cell and other optoelectronic applications for reasonsof good charge carrier transport and light harvesting effects. The insetof FIG. 3( b) shows a zoomed-in version of one of the flowers of TiO₂(TiCl₄ treated ZnOFlr).

The shape preserving transformation of ZnOFlr to TiO₂ Flr seen here mustoccur through the cation exchange reaction between Zn2+ and Ti4+ ions asanticipated and mentioned earlier. FIG. S-3 shows the energy dispersivex-ray (EDX) data for the image of FIG. 3( b) indicating a completeabsence of Zinc for the case of TiCl₄ treated ZnO consistent withcomplete ion exchange. The XPS data shown in FIG. S-4 further confirmthe complete absence of Zn in the case of TiCl₄ treated ZnO.

The FE-SEM image of hexagonal shaped ZnO rods on FTO substrate is shownin FIG. 4( a). After TiCl₄ treatment of ZnO rods on FTO, all the ZnOrods are seen to get converted to anatase TiO₂ structure of nominallysimilar shape (FIG. 4( b)) but interestingly as hollow tubes. Theschematic diagram of suggested mechanism involving conversion of ZnOrods to hollow anatase TiO₂ tubes is shown in FIG. 4( c). During theTiCl₄ treatment, all the Ti4+ ions are first adsorbed on the surface ofZnO rods and then Ti4+ ions and Zn2+ ions are exchanged slowly amongthemselves via ion exchange mechanism. There may be two possiblescenarios for this ion exchange reaction (i) the new Ti4+ ions adsorbedon ZnO rods diffuse inward continuously, resulting in a directionalmigration of the reaction interface towards the core, or (ii) The Ti4+ion diffusion is limited and core species (Zn2+) diffuse outward,generating a void space inside the rods. The Zn²⁺ ions exchanged by Ti4+ions would combine with Cl-ions present in the solution to form ZnCl₂(which is soluble in water thereby coming out as side-product) whereasthe Ti4+ ions would take the place of Zn²⁺ via diffusion to form TiO₂ asmajor product. The effect of the TiCl₄ treatment on properly aligned ZnOrods on ITO substrate is brought out in FIG. S-5.

The BET surface area data on the ZnOFlr system before and after itsconversion into anatase TiO₂ Flrs is given in Table-1. The area is seento be enhanced from about 5.9 m2/gm to 30.5 m²/g, i.e. by a factor ofabout 5. This is consistent with change from a rod to a tube structurewhich would increase the area by a factor of about 2; the extramultiplying factor being added by the roughness enhancement.

In FIG. 5( a), flake-like morphology of ZnO is shown, which was achievedby a special synthesis protocol. Very interestingly, by using the TiCl₄treatment of these ZnO flakes, flake-like TiO₂ structures could beobtained, replicating the original morphology, as shown in FIG. 5( b).

The above explanation collectively shows that the suggested treatment isnot only facile but versatile in transforming oxide phase by preservingshapes in the broad sense. Indeed even hierarchically structured ZnOmesosystem is also converted to hierarchically structured anatase TiO₂with diameter ranging from 1 μm to 2 μm by TiCl₄ treatment keeping themorphology broadly conserved, as shown in FIG. S-6.

In another embodiment, the TiO₂ mesostructures of the present inventionis used in Dye Sensitized Solar Cells (DSSC) as light harvesters becauseof enhanced light scattering in the visible region thereby enhancing thepath length of incident light within the nanocrystalline TiO₂ electrode.

In order to investigate the role of TiO₂ mesostructures obtained afterTiCl₄ treatment of ZnO mesostructures as light harvesters in TiO₂ basedDSSC, double layer structures (TZFT film) were made with 7 μm thicknanocrystalline anatse TiO₂ with an over layer (4 μm) of TiCl₄ treatedZnOFlr (implying effectively an anatase TiO₂ flower morphology). TiCl₄treated films (˜11 μm thick) with only nanocrystalline TiO₂ and thecommercial Degussa P25 without such an over-layer were also preparedunder similar conditions for comparison. All films had an active area of0.25 cm².

FIG. 6 (see also the Table 2) compares the photovoltaic characteristicof all the three cases. The optimized mean efficiencies obtained by ourprocedure for P25 and Nanocrystalline TiO₂ are about 5.2% and 5.4%,respectively. After TiCl₄ treated ZnO is introduced as an over layer,the conversion efficiency improved from 5.4% to 6.9%, a 28% increment.It is important mention here that higher efficiencies have been reportedin the literature even for the nano-TiO₂ and P25 cases using the samedye, but achieving such efficiencies requires simultaneous optimizationof several parameters and significant experience and skill in cellarchitecture design.

In this invention the emphasis is on shape preserving transformation andour attempt here is to demonstrate the possible use of such anisotropicTiO₂ architectures in device improvements; the observed enhancementbeing significant within our current skill set in device making.

It can be seen that the open circuit voltage (VOC) of TZFT film (0.78 V)is almost 18% higher than that for TiO₂ nanocrystalline film (0.66 V)and P25 film (0.67 V). Also the fill factor is higher for the case ofTZFT films (˜63%) than the case of TiO₂ film (58%) and P25 (˜60%).Increase in VOC and Fill Factor can be correlated with decreasedelectron-hole recombination at TiO₂-dye-electrolyte interface.

Decreased recombination in TZFT film can be attributed to high qualityof TiO₂ Flr (over layer) formed after TiCl₄ treatment of ZnOFlr. It hasbeen shown that the TiO₂ formed after TiCl₄ treatment has conductionband edge potential 80 mV lower than conventional TiO₂ nanoparticlesthereby causing 20 fold decreases in electron-electrolyte recombinationrate constant which is responsible for increase in Voc. No substantialincrement in JSC was observed for the case of TZFT films. The shortcircuit current density (JSC) for TZFT film is 14 mA/cm² which is aboutthe same as that for the case of the TiO₂ film (12.9 mA/cm2 for P25).This can be attributed to less dye adsorption in the TZFT films.

In order to quantify the amount of dye adsorbed measured the absorptionsof solutions containing dye (see FIG. S-7) detached from the TiO₂ andTZFT films. From FIG. S-7 calculate the dye loading of the TiO₂ and TZFTfilms, which have values of 8.9×10⁻⁸ and 4.7×10⁻⁸ mol/cm², respectively.It is interesting that although the dye loading of the TZFT film is farless (by almost 50%) as compared to the TiO₂ nanocrystalline film, stillthe current density for TZFT remains the same (˜14 mA/cm²). In order toinvestigate this aspect further measured the diffused reflectancespectra (DRS) for the TiO₂ and TZFT films without dye. It is observed(FIG. S-8 (a)) that the (diffused) reflectance of TZFT film is higherthan that of nanocrystalline TiO₂ film. This implies improved scatteringof TiO₂ Flrs (over layer) formed after TiCl₄ treatment of ZnOFlrs whichenhances the path length of light within the nanocrystalline TiO₂.

Since the DSSC systems contain dye adsorbed films, the DRS of dyeadsorbed films was recorded for gaining further insights. As shown inFIG. S-8 (b), after dye adsorption on the films the reflectance valuesfor the TZFT and the nanocrystalline TiO₂ film decrease significantly,which is mainly due to light absorption by the dye molecules. However,the dye-adsorbed TZFT film exhibits a substantially higher reflectancethan the dye adsorbed nanocrystalline TiO₂ film which is due to low dyeadsorption of the TZFT film (as discussed earlier) and the strong lightscattering effect of the over layer of TiCl₄ treated ZnOFlr on the firstlayer of TiO₂ nanoparticle (TZFT film). This shows that the decrease incurrent density due to less dye adsorption is counterbalanced by theenhancement in current density due to improved light scattering effectof TiO₂ Flr over layer in TZFT films thereby keeping current densityalmost the same. Therefore, the enhancement of efficiency in TZFT filmscan be attributed to both increase in VOC and improved light scatteringeffect within the film.

EXAMPLES

The following examples are given by way of illustration therefore shouldnot be construed to limit the scope of the invention.

Example 1

Shape preserving chemical transformation of ZnO mesostructures intoanatase TiO₂ mesostructures is prepared by providing films of ZnOmesostructures grown on FTO/ITO glass* plates and treating the saidfilms with TiCl_(a) solution at 70° C. for about 30 min, followed bywashing with D.I. water followed by annealing at 450° C. for 30 min toobtain anatase TiO₂ mesostructures. The thickness of these films is ˜5μm.

Example 2

Shape preserving chemical transformation, of ZnO mesostructures intoanatase TiO₂ mesostructures is optionally prepared by coating a layer ofZnO mesostructures over TiO₂ nanoparticles film which is subjected toannealed at 450° C. for 60 min and these films are further treated withTiCl₄ solution at 70° C. followed by second annealing at 450° C. for 30min to obtain TiO₂ mesostructures (flowers) of the thickness of 11 μm.

The first layer of TiO2 nanoparticles is ˜6 μm and on that anatase TiO2mesostructure (obtained from ZnO mesostructures) of thickness 5 μm wascoated. Therefore total thickness was 11 μm.

Example 3 Experimental Details

Materials:

The chemical agents were purchased from Aldrich Co. and Merck Chemicals.The RuL₂(NCS)₂/(TBA)₂ (N719Dye; L=2,2′-bipyridine-4,4′-dicarboxylic acidand TBA=tetrabutyl ammonium) and the fluorine-doped SnO₂ (FTO) electrode(sheet resistance 15 ohm/square) were purchased from Solaronix Co. Forthe preparation of reference DSSCs, commercial TiO₂ was obtained fromDegussa (P25). High-purity water (Milli-Q, Millipore) was used for allexperiments. The FTO electrodes were washed with acetone, ethanol, anddeionized (18.2 MΩ·cm) water in an ultrasonication bath for 15 min witha final wash in isopropyl alcohol.

Example 4 Preparation of ZnO Flowers

The ZnO flowers used in the present invention were synthesized byhydrothermal route using high purity zinc acetate and NaOH. Forobtaining ZnO flower, a 150 ml, 0.01M aqueous solution of zinc acetatewas prepared and magnetically stirred for 10 min. After the dissolutionof zinc acetate, 6 ml of 6.67 M aqueous solution of NaOH was added tothe above solution. This solution was transferred into a Teflon linedstainless steel autoclave. It was then sealed and maintained at 180° C.for 2 h. After the reaction a white colored solid powder was recoveredby centrifugation followed by washing with distilled water and ethanolto remove the residual ions in the final product. Then the powder wasfinally dried at 60° C. in air for 5 h.

Example 5

Preparation of ZnO Rods on FTO and ITO

Zinc acetate, Zinc nitrate, Hexamethylenetetramine (HMT) and SodiumHydroxide Pellets were used as precursors for ZnO rod growth. Zincacetate solution (5 mM concentration) was prepared in methanol and waskept under stirring at 65° C. for 45 min. Then sodium hydroxide solution(30 mM concentration, prepared in Methanol) was added drop wise till thesolution attained slight milky color and was used as seed solution.Fluorine doped tin oxide (FTO) and Indium doped tin oxide (ITO) glassplates (2.5 cm×2.5 cm) were used as substrates for growth of ZnO rods.The substrates were mounted on the spin coater having a preset rotationspeed of 2500 rpm for 30 sec and then spin coating was carried out usingfreshly prepared seed solution. The process was repeated continuouslyuntil the transparent substrate turned slightly opaque. Finally thesubstrates were annealed at 300° C. for 1 hr for better adherence of ZnOnanoparticles which act as nucleating sites for the growth of ZnO rods.

For facile growth of ZnO rods, equimolar solutions of Zinc Nitrate (25mM) and hexamethylene tetramine (HMT 25 mM) were separately preparedusing de-ionized water as solvent. The seeded substrates were immersedinto the solution and the solution temperature was maintained at 95° C.under slow stirring. The depositions were carried out for time durationof 3 hour. Finally, the deposits were annealed at 300° C. for 1 hr. forremoval of moisture and for improving the adhesion.

Example 6 Preparation of ZnO Spheres and Flakes

The ZnO sphere like morphology was synthesized at room temperature byco-precipitation method using zinc acetate, polyvinyl pyrrolidone andsodium hydroxide 0.02M aqueous solution of zinc acetate was prepared andmagnetically stirred for 5 minutes. 0.5 gm of polyvinyl pyrrolidone(capping agents) was dissolved in 10 ml of deionized water and added tothe zinc acetate solution. Then 10 ml of 2M aqueous solution of NaOH wasadded drop wise to the above solution. A white colored solid powder wasobtained and recovered by centrifugation followed by washing withde-ionized water. Then, the powder was finally dried at 60° C. in airfor 10 h. The ZnO flakes were obtained from a commercial source (SmartNanoZ, Pune, India).

Example 7 Preparation of TiO₂ Nanoparticles

Nanocrystalline TiO₂ was prepared by using a simple hydrothermal method.2 ml of Titanium Isopropoxide was hydrolyzed by adding 100 ml ofdeionized water and then sonicated for 5 minutes. The solution wastransferred to Teflon lined autoclave vessel along with 3 ml of H₂SO₄(1M). This autoclave vessel was kept at 175° C. for 24 Hrs. Theresulting product was washed thoroughly with deionized water and driedat 50° C. in adust proof environment to produce the powder of TiO₂nanoparticles.

Example 8

Fabrication of Dye Sensitized Solar Cell

Doctor blading method was employed to first make the TiO₂ nanoparticlefilm (thickness ˜7 μm) and then an over layer of ZnO flowers film(thickness ˜8 μm). The total thickness of the film was ˜15 μm. Aftermaking such films they were annealed at 450° C. for 60 min. Then thesefilms were treated with TiCl₄ solution (50 mM) at 70° C. followed bysecond annealing at 450° C. for 30 min. After TiCl₄ treatment, the totalthickness of the film was found to be reduced from ˜15 μm to ˜11 μm.Same thickness (−11 μm) of TiCl₄ treated TiO₂ nanoparticle and P25(Degussa) films were made for comparison. The films were impregnatedwith 0.5 mM N719 dye in ethanol for 24 h at 27° C. The samples were thenrinsed with ethanol to remove excess dye on the surface and wereair-dried at 27° C. This was followed by redox electrolyte addition andtop contact of Pt coated FTO as known in the art. The electrolyte usedwas 1M 1-hexyl-2,3-dimethyl-imidazoliumiodide, 0.05 M LiI, 0.05M I2, and0.5 M 4-tert-butylpyridine in acetonitrile. The J-V characteristics weremeasured by exposure to 100 mW/cm² (450W xenon lamp, NewportInstruments), 1 sun AM 1.5, simulated sunlight by a solar simulator. Thecurrent was measured using a Kiethley 2420 source meter.

Example 9 BET Surface Area Data

The BET surface area data on the ZnOFlr system before and after itsconversion into anatase TiO₂ Flrs is given in Table-1. The area is seento be enhanced from about 5.9 m2/gm to 30.5 m²/g, i.e. by a factor ofabout 5. This is consistent with change from a rod to a tube structurewhich would increase the area by a factor of about 2; the extramultiplying factor being added by the roughness enhancement.

TABLE 1 The BET Surface area measurements of ZnO Flr and TiCl₄ TreatedZnO Flr. Name Surface Area(m²/g) ZnO Flr 5.9 TiCl₄ treated ZnO Flr 30.5

Example 10 Comparison of Photovoltaic Properties of DSSC

TABLE 2 Photovoltaic properties of dye-sensitized solar cells (DSSC)Fill Jsc Factor Efficiency Name Voc (V) (mA/cm²) (FF) (η)% Degussa P250.67 12.9 60.5 5.2 TiO₂ 0.66 14.0 58.1 5.4 1^(st) layer TiO₂ + 2^(nd)layer 0.78 14.0 62.8 6.9 TiCL₄ treated ZnO Flr (Example 2) The thicknessof all films of DSSC were 11 μm. TiO2 mesostructure 0.79 7.5 58.5 3.5from Example 1

In Conclusion, ZnO mesostructures (rods, spheres, flakes and flower-likemorphologies) are converted to anatase TiO₂ mesostructures by a simpleTiCl₄ treatment and this process exhibits a remarkable nominallyshape-preserving property. Thus, for the case of ZnO flowers andspheres, anatase TiO₂ flowers and spheres are obtained, respectively,albeit with small changes in morphology details. Interestingly anataseTiO₂ hollow rod like structures are obtained by TiCl₄ treatment of ZnOrods. Post-treatment appearance of Raman peaks at 148.2, 401, 518 and642 cm⁻¹ that are the characteristics of pure anatase TiO₂ phase clearlyindicates the complete conversion of ZnO structures to anatase TiO₂. Itis observed that the morphology conversions of ZnO to TiO₂ are due tothe ion exchange reaction i.e. between Zn²⁺ and Ti⁴⁺. These convertedTiO₂ mesostructures are used for light harvesting to absorb more photonsfrom sunlight in Dye-sensitized Solar Cells for better conversionefficiency.

Advantages of the Invention

The present process provides synthesis of shape controlled nanomaterials(anatase TiO₂) by an easy and economical process

The present process provides anatase TiO₂ mesostructures without usingany selective capping agent.

1. A process for the shape preserving chemical transformation of ZnOmesostructures into anatase TiO₂ mesostructures comprising the steps of:i. treating the Zinc oxide mesostructures with Titanium tetrachloride(TiCl₄) solution at temperature in the range of 60 to 70° C. for periodin the range of 20 to 30 min; ii. annealing the TiCl₄ treated Zinc oxidemesostructures as obtained in step (i) at a temperature in the range of400 to 450° C. for period in the range of 20 to 30 min to obtain anataseTiO₂ mesostructures.
 2. The process of claim 1, wherein the Zinc oxidemesostructures are selected from the group consisting of Zinc oxiderods, Zinc oxide spheres, Zinc oxide flakes and Zinc oxide flowers. 3.The process of claim 1, wherein the Zinc oxide mesostructures are coatedover Titanium dioxide nanoparticles film and annealed at a temperaturein the range of 400 to 450° C. for 50 to 60 min before treating withTitanium tetrachloride (TiCl₄) solution.
 4. The process of claim 1,wherein the Zinc oxide mesostructures are optionally grown on Fluorinedoped Tin oxide (FTO) or Indium doped Tin oxide (ITO) glass platesbefore treating with Titanium tetrachloride (TiCl₄) solution.
 5. Theprocess of claim 4, wherein the Zinc oxide mesostructures treated withTitanium tetrachloride (TiCl₄) solution are washed with deionized water.6. The process as claimed in claim 1, wherein the thickness of anataseTiO₂ mesostructures is in the range of 5-12 μm.
 7. The process asclaimed in claim 1, wherein the diameter of anatase TiO₂ mesostructureis ranging from 500 nm to 2 μm.
 8. Anatase Titanium dioxidemesostructures prepared by the process of claim
 1. 9. Anatase Titaniumdioxide mesostructures as claimed in claim 8, wherein saidmesostructures are useful for optoelectronic applications.
 10. AnataseTitanium dioxide mesostructures as claimed in claim 8, wherein the dyesensitized solar cells utilizing said mesostructures exhibit efficiencyin the range of 3.5% to 7%.